Transmission control in two-way signaling systems



Jan. 8, 1946. A, Q N RwmE 2,392,496

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Sept. 5, 1942SSheets-Sheet 1 I F/a/ I J a, I EA 4J 1 f ES, I

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(c) (b) l I l l (a) DESE'NSlT/ZATION (IN DEC/EELS) INPUT TO ECHOSUPREJSOR (IN DEC/EELS) INVEN TOR A C. NORW/NE AT T ORNE V Jan. 8, 1946.A. c. NORWINE ,3

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Sept. 5, 1942 3Sheets-Sheet 2 v INVENTOR A. C NORW/NE ATTORNEY Jan. 8, 1946. v A. c.NORWINE 9 TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Sept.5, 1942 5 Sheets-Sheet 3 FIG. 4 25 /'A 057. I [27 32..- 057. NE

A 7' TORNE V Patented Jan. 8, 1946 TRANSMISSION CONTROL IN TWO-WAYSIGNALING SYSTEMS Andrew C. Norwine, Short Hills, N. J., assignor toBell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application September 5,1942, Serial No. 457,393

6 Claims.

The invention relates to two-way signaling systems, such as two-waytelephone systems, and particularly to the signal-controlled switchingcircuits employed with such systems to direction- "ally control signaltransmission therein while suppressing echoesand preventing singing.

Such signal-controlled switching circuits, commonly called echosuppressors, usually comprise two switching branches respectivelyconnected to each of the two oppositely directed one-way signaltransmission paths at a'terminal or intermediate point of the two-waysystem, each including a rectifier for rectifying the portion of thesignal waves diverted into the switching branch, and electromagneticrelays or other switching devices operated by the rectified signalstosuitably control the transmission efiiciencies of the signaltransmission paths and to disable the other switching branch, so as toprovide the desired directional control of signal transmission whilesuppressing echoes and preventing singing.

The prior patent application of K. H. Davis and A. C. Norwine, SerialNo. 421,704, filed December 5, 1941, which issued as Patent No.2,306,689 on December 29, 1942, discloses such an echo suppressorapplied to a four-wire telephone circuit,

modified so as to enable a subscriber at one terminal of the system bytalking loudly to break in on another who is talking and has obtaineddirectional control of the circuit, without waiting for the latter topause and regardless of how loudly the latter subscriber is talking.This is accomplished by the use of relatively insensitive auxiliaryswitching controls, properly located with respect to the echo suppressorcontrols, operating on an amplitude basis to disable the echo suppressorcontrols of the talking subscriber when the breaking subscriber talkswith the required loudness. These auxiliary controls necessarily have tobe relatively low in sensitivity to above-described type, required forbreak-in, without increasing the probability of false operation byechoes.

These objects are attained in accordance with one embodiment of theinvention by adding means to desensitize the break-in controls connectedto each one-way transmission path in resented by the box. An arrowthrough the rep-.

proportion to the loudness of the speech signals in the oppositelydirected one-way transmission path, so that a "loud talker needs to talkonly slightly louder than the echoes of a "weak" talker who has obtaineddirectional control of the system, to break in immediately.

In other embodiments of the'invention echo suppression is obtained andbreak-in facilitated by providing limited desensitizations of eachone-way signal transmission path and the echo suppressor control branchcontrolled from that path, or, alternatively, of each signaltransmission path on either side of the point of connection of the inputof the associated echo suppressor control branch, which are differentfunctions of the amplitude of the signals transmitted over theoppositely directed one-way signal transmission path controlling theother echo suppressor control branch.

The various objects and features of they invention will be betterunderstood from the following complete description when read inconjunction with the accompanying drawings in which:

Fig. 1 shows a functional schematic of a fourwire telephone circuitequipped with an echo suppressor of the amplitude break-in typeembodying one modification of the invention;

Fig. 2 shows schematically one detailed circuit arrangement foraccomplishing the functions diagrammatically illustrated inFig. 1;

Fig. 3 shows curves used for explaining the different types of operationwhich may be obtained with circuit arrangements such as illustrated inFigs. 1 and 2; and

Figs. 4 and 5 show schematically, and in part diagrammatically, othermodifications of the invention. Y

In the functional diagram of Fig. 1, each single line represents atwo-wire signal transmission path. Contacting arrowheads at a point in atransmission path indicate that the path is normally enabled at thatpoint, and an arrow pointing from a box towards the contactingarrowheads indicates that the path will be disabled at that point byoperation of the apparatus representation of a resistance at a point ina transmission path, pointing from a box indicates that the sensitivityof the path at that point will be adjusted by operation of the apparatusrepresented by the box.

The four-wire telephone circuit of Fig. 1 comprises a one-way, two-wiretransmission path EA normally conditioned to transmit telephone siglsthe direction from west to east between the two-way line WI.- and thetwo-way line EL,

' and a one-way, two-wire transmission path WA suppressor branch ESioperating in response to applied telephone signals from the path EA,when that path is operative, to disable the east-to-west transmissionpath WA at the intermediate point D. Similarly, connected across thepath WA at a. point beyond the disabling point I) in that path is theinput of a second echo suppressor branch ES: operating in response toapplied signals from the path WA, when that path is operative, todisable the path EA at a point (1 in front of the point a to which theecho suppressor ESi is connected. Thus, the echo suppressor branchconnected to the one-way path of the tour-wire circuit transmitting thesignals of the subscriber at one terminal of the system, who is first totalk, operates to disable the oppositely directed one-way path andeflectively the associated echo suppressor branch so as to giveexclusive directional control of the system to the first talker.

Connected across the transmission path EA at a point e infront of thedisabling point at in that path is the input of a break-in controlcircuit 3K1, relatively insensitive compared to the echo suppressorbranch ESi, operating in response to applied telephone signals from thepath EA of sufilcient amplitude to overcome its desensitization todisable the echo suppressor branch ES; controlled from the path WA, atthe point 1 in its output. Similary, connected across the transmissionpath WA at a point g in front of the disabling point D is the input of abreak-in control circuit BKz, relatively insensitive compared to theecho suppressor branch ESz, operating in response to telephone signalsfrom the path WA of sum-- cient amplitude to overcome itsdesensitization to disable the echo suppressor branch ES1 connected tothe path EA at a point h in its output. As described in detail in theaforementioned Davis- Norwine patent application disclosing similar-incontrol circuits, after the subscriber at one terminal of the system whofirst starts to talk, has operated the echo suppressor branch associatedwith the one-way path EA or WA transmitting his signals, to seizedirectional control of the system in the manner described above, andwhile that subscriber continues to talk, a subscriber at the otherterminal of the system by talking loudly enough to cause operation ofthe break-in control branch fed from the oppositely directed one-waypath which transmits his signals, may disable the echo suppressor of thefirst subscriber to interrupt the directional control of the circuit bythe latter so as to allow the signals of the second subscriber to getthrough to the first subscriber. If the first subscriber continues totalk with normal loudness, the second subscriber by operating his ownecho suppressor branch may disable the oppositely directed transmissionpath to seize directional control of the system and cut off the firstsubscriber, but if the first subscriber also talks with sufficientloudness to operate his own break-in control branch to disable the echosuppressor branch of the other subscriber, both subscribers will be ableto hear each otherbut there will be no echo suppression.

As noted above, a disadvantage of break-in control echo suppressors ofthe type just described is that the auxiliary break-in controlsnecessarily have to be relatively low in sensitivity to avoid falseoperation by echoes of the loudest possible speech. In accordance withthe present invention the loudness ot the second talker required forbreak-in is made substantially smaller without increasing theprobability of false operation by echoes, thus improving the naturalnessof conversation. This is accomplished, as indicated diagrammatically inFig. 1, by adding means for causing the break-in control branchcontrolled from each one-way transmission path to be desensitized inproportion to the amplitude of'the speech signals in the oppositelydirected one-way transmission path. Thus a loud talker needs totalk'only slightly louder than the echoes of the wea talker who hasobtained directional control or the system to break in immediately. Thisdesensitization may be accomplished as indicated diagrammatically inFig. 1, by adding controls DCl, DC: to adjust a variolosser VL1 or VLcin the'input of the break-in control branch BKi, BK: connected to eachoneway transmission path 'in accordance with the amplitude of therectified signalsin the output of the echo suppressor branch ES: or ES:controlled from the oppositely directed one-way path, or, alternatively,the desensitization controls may be controlled by an auxiliary detectorconnected to the oppositely directed one-way path (as illustrated inFig. 2). The hang-over of each variolosser VLi, VLo should be made suchthat the desensitization of the break-in control connected to each pathin response to each controlling signal in the other path will bemaintained for a suflicient time interval to prevent false operation ofthe control by the echo of that signal.

Various types of operation which may be obtained with the desensitizingarrangements of the invention as shown in Fig. 1 (or the alternativearrangements of Fig. 2) depending on the adjustment of the desensitizinggaincontrols are illustrated in the curves of Fig. 3, in which thedesensitization (in decibels) of a break-in control branch controlledfrom one side of the four-wire circuit is plotted against the input (indecibels) to the echo suppressor branch controlled from the oppositelydirected side of the four-wire circuit. Curve a shows limited rangedesensitization, that is, the break-in control operated from one side ofthe four-wire circuit is desensitized in direct proportion to theloudness of signal input to the echo suppressor branch operated from theother side. up to the knee of the curve; beyond that, thedesensitization (and thus the break-in control sensitivity) is constant.With this adjustment, echo suppressor characteristics which may beidentified by the code letters NBE are obtained. The first code letter Nof the three means that the echo suppressor provides normal echosuppression if both talkers are weak or only one talker is speaking. Thesecond letter B of the code means that if only one of the talkers isloud enough to operate his break-in control a normal break-in occurs andhe gains directional control of the system. The third letter E in thecode mean that if both talkers talk loudly enough to operate theirbreak-in control both echo suppressors are lifted off so that bothtalkers hear each other, and double talking echoes occur. With thisadjustment. the initial breaking sensitivity can be increased by therange of desensitization, if the desensitization controls are providedwith sufficient hangover in operation by any suitable means to pretheecho of the weak talker to be able to break in immediately.

The range of input over which desensitization is changing determines therange for which the middle letter B" of the code will hold. That is,both talkers will have to talk louder than the input defining the kneeof thecurve a to reach the condition referred to by the last code letterE. In the case of curve b, there is no range where this condition can bereached, so that we have an NB echo suppressor which is similar to theordinary differential echo suppressor except that breaking will befaster because the hang-over of differential is applied to desensitize,not to dis-- able, the opposite side of the four-wire circuit. With theadjustment of curve c, the break-in controls are disabled for inputshigher than the knee of the curve, so that the suppressor is changed towhat may be called the NBN type, that is, one that provides normalsuppression for weak talkers or only one talker; if one of the talkersis loud enough to operate his break-in control.

a normal break-in occurs and he gains control of the echo suppressor;and when both talkers talk loudly enough to operate their break-incontrols. the echo suppressor functions normally to give directionalcontrol to the first talker to speak.

To make the above-described types of break-in effective, it is necessarythat the relative sensitivities of the various echo suppressor controland the relative normal sensitivities of the breakin controls beproperly proportioned.

Fig. 2 shows in more detail the circuit; and apparatus which could beused to accomplish the echo suppressing, break-in control anddesensitizing control functions more diagrammatically illustrated inFig. 1. The four-Wire telephone c rcuit of Fig. 2 includes the one-waytransmission circuit EA including the amplifier AE, normally conditionedto transmit in the direction from east to west, and the one-waytransmission circuit WA including the one-way amplifier Aw, normallyconditioned to transmit in the direction from east to west. The input ofthe circuit EA and the output of the circuit WA may be coupled at tl'ewest terminal of the four-wire circuit by the usual hybrid coil andassociated line balancing network (not shown) to a west two-way lineleading to subscribers stations, and the output of the circuit EA andthe input of the circuit WA may be coupled at the east terminal of thefour-wire circuit by the usual hybrid coil and associated line balancingnetwork (not shown) to an east two-way line leading to subscribersstations.

Connected across the circuit EA at an intermediate point I is the inputof an echo suppressor branch circuit 2. to be referred to hereinafter asthe west echo suppressor, including the wave detector 3 and theelectromagnetic relay 4 adapted to be operated in response to therectified voice signal output of the detector 3. Similarly, connectedacross the east-to-west circuit WA at an intermediate point 5 is theinput of an echo suppressor branch circuit 6, to be referred tohereinafter as the east echo suppressor, including the A point 9 infront of the point 5 to which the input of the east echo suppressor Iiis connected, 'to disable that circuit, and simultaneously to close ashort-circuiting connection across the input of the detector 1 in theecho suppressor 6 to disable iii) that echo suppressor. Similarly, therelay 8 of the east echo suppressor 6 operates to close ashort-circuiting connection across the circuit EA at the point III infront of the point I to which the input of the west echo suppressor 2 isconnected to disable that circuit, and simultaneously to close ashort-circuiting connection across the input of detector 3 in the westecho suppressor 2 to disable that echo suppressor,

Connected across the west'-to-east transmission circuit EA at a point Hin front of the disabling point I0 therein is the input of a thirdcontrol circuit [2 relatively insensitive compared to the west echosuppressor 2, which will be referred to hereinafter as the west break-incontrol, including the wave detector I3 and electromagnetic relay l4adapted to be operated in response to operation of the detector [3 byapplied voice signals of sufilcient amplitude to overcome thedesensitizing loss in its input, to put a disabling break in theshort-circuiting connection across the circuit EA at the point it, andthus to effectively-disable the echo suppressing function of the eastecho suppressor 6.-

Similarly, connected across the east-to-west circuit WA at a point i5 infront of the disabling ,pointS is the input of a control circuit l6relatively insensitive as compared to the west echo suppressor 6, whichwill be referred to hereinafter as the east break-in control, includingthe wave detector ll and the electromagnetic relay l8 adapted to beoperated by operation of the detector in response to applied voicesignals,

in front of the disabling point I0 is the input of a fifth controlcircuit 20, to be referred'to hereinafter as the east break-in controldesensitizer, including the wave detector 2 I, which may be of theelectron discharge tube type as indicated, and

a plurality of marginal electromagnetic relays R1, RN of differentsensitivities ranging from a high sensitivity for relay R1 to a lowsensitivity for the relay RN, connected in series across the output ofthe detector 2|. The highest sensitivity relay R1 is adapted to beoperated by the rectified signal output of the detector 2| in responseto the amplitude of voice signals applied to the input of the detector2| which would be produced by the weakest direct voice signal (notechoes) passing over the circuit EA; the lowest sensitivity relay RN isadapted to be operated by the rectified voice signal output of thedetector 2| in response to an amplitude of the voice signals applied tothe input of the detector 2| which would be produced by the loudestreceived voice signals passing over the path EA; and the other relays,R2, R3 are adapted to be respectively operated by different rectifiedsignal outputs of the detector 2| in response to amplitudes of the voicesignal inputs to'that detector produced by voice signals of differentintermediate stagesof loudness passing over the circuit EA. TherelaysR1, R2, R3 Ru when operated respectively connect the losses L1, L2, L3LN across the input ofthe detector H in the west break-in control IS.The value of the loss L1 is selected such that when it is connectedacross. the input of the west break-incontrol 16 it will reduce thesensitivity of that control by an amount sufficient to sert the lossesL11, L12, L13

Just prevent false operation 01' that control by echoes applied from thepath WA of the signals causing operation of the relay R1. Similarly, thevalue of each of the other losses L2. L: Ln is selected so that wheneach in turn is connected across the input of the break-in control ISthe total loss in the input of that control will reduce its sensitivityby an amount just sufficient to prevent false operation of the controlby echoes of the signals of the amplitude causing the operation of therespective relays which are operated. Also, each of the relays R1, R2,R3 Rn is provided with the same hang-over in operation, which issufilcient to maintain the loss which it controls connected in the inputof the west breakin control I6 for just suflicient time to prevent falseoperation of that control by the echo of the signal causing theoperation'of the relay. The number of relays used in the chain R1 Rswill depend on the range of speech volumes for which desensitization isrequired and the decibel loss inserted per relay. 1

Similarly, connected across the circuit WA at a point 22- in front ofthe disabling point 9 is the input of a sixth control circuit 23, to bereferred to hereinafter as the west break-in control desensitizer,including a wave detector 24, like detector 2i, and a plurality ofmarginal relays R11, R12, R13 Rm of different sensitivities covering thesame range as the relays R1 RN in the west break-in control desensitizer20 andwith hang-overs computed in the sameway, connected in seriesacross the output of the wave detector 24 and operating in response toapplied voice signals from the circuit WA of difierent amplitudes overthe desired range of control, respectively in- Lin, equivalent to thelosses L1, L2, L3 LN in the west break-in control desensitizer 20,across the input of the detector l3 of the west break-in control l2, toprevent false operation of the west break-in control l2 by the echoes ofthe signals controlling the operated relays.

The circuit arrangement of Fig. 2 operates as follows:

Let it be assumed that a west telephone subscriber at the west terminalof the four-wire circuit starts to talk, and at that time the eastsubscriber at the east terminal of the four-wire circuit is silent.Wests speech waves will pass over the circuit EA to the east subscriberbeing amplified en route by the amplifier An. A portion of Wests signalswill be diverted from the circuit EA at the point ll into the input ofthe west break-in control l2, which, atthe time, with no speech. wavesbeing received from the east subscriber over the circuit WA, will be inits most sensitive condition so that at least a portion of Wests speechsignals may be'of high enough amplitude to momentarily cause operationof the break-in detector l3 and thus of relay M to put a break in theshort-circuiting connection across the circuit EA at the point ID. Thiswill have no effect on the directional control of transmission becausethat short-circuit-ing connection is broken at another point with theeast echo suppressor unoperated.

Another portion of Wests speech signals will be diverted from thecircuit EA at the point l9 into the east break-in control desensitizer20 and will cause operation of the detector 2| therein to operativelyenergize one or more of the chain of relays R1 Rn in its output,depending on the loudness of Wests speech, to cause one or more of thelosses L1 LN to be inserted into the input of'the detector ll of theeast break-in control Hi to just sufliciently desensitize that'controlso as to prevent its false operation by applied echoes from the circuitWA or Wests signals causing the operation of the operated relays inthechain R1 RN.

Another portion of Wests speech waves will be diverted from the circuitEA at the point I into the west echo suppressor 2 and will cause theoperation of the detector 3 and thus of the electromagnetic rela 4 toclose the short-circuiting com nection across the circuit WA at thepoint 9 to disable that circuit, and to close a second short- Now, ifthe east subscriber desired to break in on West, the former will raisehi voice so that at least a portion of his speech signals transmittedover the circuit WA-and diverted into the east break-in control 1.6 willbe of sufiicient amplitude to overcome the desensitization of thatcontrol produced by West by operation of the east breakin controldesensitizer 20 in a manner which has been described. As thedesensitization of the east break-in control 16 is only that which willjust prevent false operation by the echoes of Wests speech signals, itwill be apparent that East need only talk slightly louder than theechoes of Wests speech signals to accomplish this. If East does so, hisspeech waves will cause operation of the detector I1 and thus of theelectromagnetic relay While West continues to talk, however, his voicesignals will maintain the relay 4 in the west echo suppressor 2 operatedto hold the east echo suppressor 6 disabled in its input. Thus, bothtransmission circuits WA and EA will be operative to respectivelytransmit the signals of the west and east subscribers to the othersubscriber, but there will be no suppression of echoes.

Another portion of Easts speech waves will be diverted at the point 22into the west break-in control desensitizer 23 and will cause theoperation of the detector 24 and thus of one or more of.the relays R11Rmin its output, depending upon the amplitude of East's signals, toconnect one Or more of the losses L11 Lm into the input of the westbreak-in control l2. The losses thus inserted into that control willdesensitize that control just sufliciently to prevent its falseoperation by echoes from the pathEA of East's signals which produce theoperation of the operated relays of the chain R11 Rm.

Now, if West also raises his voice sufiiciently to overcome thedesensitization produced in Wests break-in control I2 his voice signalswill cause operation of the detector l3 and thus of relay M to put abreak in the short-circuiting connection across the circuit EA- at thepoint ill, but this will be inefiective to cause any change in theoperation of the circuit for that short-circuiting connection is alreadyopened because of the non-operation of relay 8 of the east echosuppressor 6 held disabled in its input by the west echo suppressor 2.

This condition of the circuits will continue until one of thesubscribers stops talking so as to release his echo suppressor and hisbreak-in control. This will allow the other talker by oper ating hissuppressor to block the echo path and disable the first subscriber'secho suppressor, to seize directional control of transmission in thesystem, and by continuing to operate the desensitization control for thefirst subscriber's break-in control making it possible for the latter tointerrupt by talking just slightly louder than the first subscriber'sechoes. V

An echo suppressor of the proportional losser type, for example, such asshown in A. B. Clark Patent No. 1,821,928, issued September 8, 1931, canbe made to suppress echoes well by inserting losses in the echo pathwhich increase several I decibels for each decibel increase in signal inthe oppositely directed path, or it can be made to permit the signal inone path, it loud enough. to interrupt the signals in the other path ifthe inserted losses increase but slowly with increases in signalamplitude. In such an .echo suppressor, the requirement of good echosuppression is incompatible with the requirement that break-in be easy.

Fig. e shows a proportional losser type of echo suppressor in accordancewith the invention which meets both break-in and. echo suppressingrequirements. Referring to Fig. i, the four-wire telephone circuitconsists of one one-wamtwowire transmission path EA including theone-way amplifiers An, normally conditioned for transmitting voicesignals from west to east, and a second one-way, two-wire transmissionpath WA including the amplifiers Aw for transmitting voice signals inthe direction "from east to west. These Paths are associated at theirterminals with a west two-way line WL and an east two-way line EL bytheusual hybrid coil and associated line balancing network.

Connected across the transmission path EA at an intermediate point 25 isthe input of'a combined echo suppressor and break-in control circuit itincluding the wave detector 2'! and connected in parallel across theoutput of the latter, the variable loss devices 28 and 29, the lossvalues of which are adjusted in accordance with the amplitude of thesignal output of the detector 21, as shown diagrammatically. Similarly,connected across the transmission path WA at an intermediate point 30 isthe input of acombined echo suppressor and break-in control circuit 3|including the wave detector 32 and the variable loss devices 33 and 34connected in parallel across the output of the latter, the 1058 valuesof which are adjusted in accordance with the amplitude of the signaloutput of the detector 32, as 11- lustrated diagrammatically. Thevariable loss -device 23 controlled from the output 01' detector 21 sothat its loss determines the sensitivity of that circuit.

The normal loss values 01' the variable loss do vices 2B and 38 in thepaths WA and EA, respectively, are made very low so that the normaltransmission efilciencies or those paths are high. Also, the normal lossvalues'of the variable loss devices 34 and 259 in the inputs of thecontrol cincuits 2G and 3!, respectively, are also made very low beingonly large enough to prevent false operation'oi. the detectors in thosecircuits in response to ,line noise from the associated signaltransmission paths.

Now, let it be supposed that a west telephone subscriber associated withthe line WL starts to talk first. Because of the normal low losscondition of the variable loss device 33 in the path EA the westsubscriber's speech waves will be transmitted over that path to the lineEL leading to the east subscriber with maximum transmission efficiencybeing amplified by the amplifiers As: on route. A portion of West'sspeech waves will be diverted from the path EA into the control circuit26 and, the variable loss device 34 therein being in its normal low losscondition, with no spee h signals of the east subscriber being passedover the path WA, will quickly cause the operation of the detector 2! toproduce an increase in loss of the variable loss device 28 in. the pathWA, to reduce transmitted echoes of West's speech waves reflected intothat path at the east terminal of the four-wire circuit. The operationof detector 21 will also cause an increase in the loss of the variableloss device 29 in the input of control circuit 3i to desensitize thatcircuit, so as to prevent its subsequent operation by applied speechwaves oi the east subscriber of normal loudness subsequently receivedfrom the line EL to reverse directional control of the circuit.Similarly, if the east subscriber associated with the line EL starts totalk first, the variable loss device 28 being in its normal low losscondition, his speech waves will be transmitted over the path WA to theline WL leading to the west subscriberwith maximum efliciency. Theportion of West's speech waves diverted into the control circuit 3!, thevariable loss device 28 therein being in its low loss condition, willquickly oper ate the detector 32 which in turn will cause the loss ofthe variable loss device 33 in thepath EA to be increased so as toreduce the transmitted echoes of East's speech waves reflected from thewest terminal of the four-wire circuit into the path EA. The operationof detector 32 will also cause the loss of the variable loss device asin the control circuit 26 to be increased thereby desensitizing thatcircuit so as to prevent its false operation by subsequently appliedspeech waves of the west subscriber of normal loudness subsequentlyreceived over the line WL and the path EA;

The variable loss devices 28 and 33 in the transmission paths WA and EA,respectively, would preferably be arranged so that the losses introducedby them in the respective transmission paths WA and EA in response tooperation of the control circuit 26 and the control circuit its lossdetermines the sensitivity of the latter 7 3!, respectively, will bejust large enoughso that circuit. Similarly, the variable loss device 33controlled from the output of detector 32 is connected in the path EAbeyond the point 25o! connection of the control circuit 26 so that itsfor each value of input signal applied to the controlling circuit theecho of that signal will be reduced to a commercial amount. On somecommercial four-wire telephone circuits this may loss is effective inthat path only, and the varia- 7o requi so large a loss for all inputsthat we have ble loss device 34 is connected in the input of the controlcircuit 28 in front of the wave-desimply a .proportionally desensitizedecho suppressor. However, if the loss introduced by the :variable lossdevice 28 or 33 in response to operation of the associated controlcircuit can be limitedand still suppress echoes, or if it is made "tofollow the syllable variations of the speech signals and thesevariationsare delayed in application to match the delay oi echoes, aloud talker could be heard through it even before his speech signals 33in the transmission path is reduced, the continuing break-in speechbecomes gradually louder, as heard by the talker being interrupted. Forthis reason, it may be less annoying than abrupt break-in, such as thataiforded by relay type break-in arrangements.

The variable loss devices 34 and 23 in the control circuits 26 and 3|,respectively, may be arranged so that when they are adjusted by thecontrol detector 32 or the control detector 21, respectively, theirlosses do not increase faster than the input to the opposite side of thecircuit, for this amount 01' desensitization will protect against falseoperation on echoes.' Also, the losses inserted need not exceed thevalue which will protect against the highest amplitude echo. Thisassumes, of course, that there is eflective hang-over in thedesensitizing control equal to the delay time of the worst echo to beprotected against.

Just as in the case of the desensitization applied to amplitude break-incontrols, it is important to designate the point at which hangover isapplied. It is intended that the hangover of proportional sensitivityreduction should mean the maintenance during the hang-over period of adesensitizing loss sumcient to prevent false operation on echo. Twothings are to be noted about this definition: first, that thedesensitizing loss reduces the sensitivity of the oppo-.

sitely directed device rather than totally disabling it, and, second,that an input louder than the postulated echo can operate thereduced-sensitivity device immediately.

The theory underlying the design of echo suppressors of the amplitudebreak-in type is that loud echo might be permissible if one or bothtalkers on a circuit are talking loudly. In a somewhat similar way, itmay be said that the divided function proportional losser echosuppressors of Figs. 4 and 5 are based upon the v theory that echosuppressing losses may be re-' duced during the. period when bothtalkers are x that of Fig'. 4 only in that the desensitizing variableloss device 34 for the control circuit 26 is placed in the path EA infront of the point to ,which the control circuit 23 is' connected, in

stead of in the control circuit 26, and the desensitizing variable lossdevice 29 for the control WA, respectively, may b arranged to insert anecho suppressing loss in these paths smaller than that which would benecessary in the circuit of Fig. 4, by the amount of the desensitizingloss for the associated control circuit.

The variable loss devices 28, 23, 33 and 34 in l the circuits of Figs. 4and 5 may be or the relayresistance type used for desensitizing thebreakin control circuit in the circuit of Fig. 2.

Various modificationsof the circuits illustrated and described which arewithin the spirit-and scope of the invention will be apparent to personsskilled in the art.

What is claimed is:

1. In combination in a two-way telephone transmission system, twooppositely directed oneway signal transmission paths, one switchingdevice. connected to each path responsive to voice signal transmissiontherein, in the absence of prior voice signaltransmission in the otherpath, to insert an echo suppression loss in the other path, and toeffectively disable said one switching device connected to said otherpath, a second relatively insensitive switching device connected to eachpath in front of the echo suppression point therein, operating inresponse to applied voice signals from the connected path of sumcientamplitude to overcome its desensitization, to rendersaid one switchingdevice connected to the other path inefiective to disable the firstpath, so as to allow break-in, and means to desensitize said secondswitching device connected to each path in-proportion to the amplitudeover a given range of amplitudes of the voice signals transmitted overth other path, to enable operation of the latter device by the lowestamplitude voice signals in the connected path consistent with preventingits false operation by echoes of the voice signals in said other path.

2. In combination in a two-way telephone system including oppositelydirected one-way transmission paths for the signals for oppositedirections, one echo suppressor device connected to each path,responsive to voice signal transmission of the subscriber at one end ofthe system transmitted over that path, in the absence of priorvoicesignal transmission of a subscriber at the other end of the system inthe other path, to disable said other path and said one echo suppressordevice connected thereto, a relatively insensitiv break-in controldevice connected to each path at a pointin front of the disabling pointtherein, responsive to applied voice signal transmission of suflicientamplitude, to efiectively disable the transmission path-disablingfunction of the echo suppressor devic controlled from the other path,.and means, to desensitize the break-in control device connected to eachpath in accordance with the amplitude level of .the voice signaltransmission, over a given range of amplitudes, applied to the echosuppressor applied voice signals above normal level consist-- ent withpreventing false operation thereof by applied echoes of the signalstransmitted over said other path in the absence of direct voice signaltransmission in the first path.

3. The system of claim 2 in which the ranges of densitization of saidbreak-in control devices are selected so that if the voice signals ofthe talking subscribers at both ends of the system are weak or if thesubscriber at one end only is talking, signal transmission over thesystem is efiected only in one direction with the echoes of that signaltransmission in the opposite direction sup pressed; if the signals ofone talking subscriber are loud and those of the other weak, the loudtalker will obtain directional control with suppression of his signalechoes; and if both talkers are loud the signals of each will betransmitted to the other subscriber but there will be no echosuppression until one subscriber stops talking.

4. In combination in a two-way telephone system, two oppositely directedone-way transmission paths for respectively transmitting the oppositelydirected signals of th subscribers at opposite terminals of the system,one switching device connected to each path, operating in response tovoice signal transmission therein, in the absence of prior voice signaltransmission in the other path, to insert an echo suppression loss insaid other path and to desensitize said one switching device controlledtherefrom, so as to give directional control of the system to thesubscriber who first starts to talk, and means to facilitate breakin byth subscriber at one terminal of the systern when the subscriber at theother terminal is talking and has obtained directional control of thesystem, and to avoid undesirable operation characteristics otherwiseobtained with loud talking by the subscribers, comprising means'to makethe echo suppression loss inserted ineach transmission path by operationof the switching device connected to the other path, to vary inproportion to the amplitude of the voice signals in said other pathcontrolling the operation of that switching device, and to make thedesensitization of th switching device connected to each path. byoperation of the other switching device connected to the other path tovary in accordance with another function of the amplitude of the signalsin said other path controlling the operation of said other switchingdevice, such as to allow operation of the first switching device byvoice signals applied from the first path of the lowest amplitudeconsistent with preventing its false operation by applied echoes of thevoice signals transmitted over said other path.

5. In combination with a two-way telephone transmission system includingtwo two-way line sections terminating at widely separated points in saidsystem and 'two oppositely directed oneway transmission paths forrespectively repeating voice signals in opposite directions between saidtwo-way line sections, two voice-operated switchin: devices respectivelyconnected to a different one of said paths at points in said systemrelatively close together, means responsive to operation of theswitching device connected to each path under control of voice signalstransmitted over the path to which it is connected, in the absence ofprior voice signal transmission in the opposite direction in the otherpath, to insert a variable loss in said other path beyond the point ofconnection thereto or the other switching device, which is at any timejust sufficient to reduce in said other path the transmitted echoes ofthe controlling signals to a predetermined tolerable amount, and othermeans responsive to operation of the switching device connected to eachpath to provide a desensitization of the other switching device varyingwith the level of the voice signals in the first path controlling thefirst switching device, and such as to permit operation of said otherswitching device by applied voice signals from said other path, of thelowest amplitude level consistent with preventing its false operation byechoes of the signals transmitted over said first path, so as tofacilitate break-in. I

6. In combination with a two-way telephone transmission system includingtwo two-way line sections terminating at separated points in said systemand two oppositely directed one-way transmission paths for respectivelyrepeating voice si nals in opposite directions between, said two-wayline sections, two voice-operated switching devices respectivelyconnected to different ones of said two paths and means responsive tooperation of either switching device under control of voice signalstransmitted over the path to which it is connected, in the absence orprior voice signal transmission in the other path, to insert a variableloss in said other path on each side of the point of connection of theother switching device thereto, which losses are different functions ofthe amplitude level of the signals in the first path controlling theoperation of the first switching device connected thereto, the sum of'the losses thus inserted in said other path being limited to that valuewhich will reduce the transmitted s18- nal echoes therein to apredetermined tolerable amount, and the loss inserted in said other pathin front of the point of connection of said other switching devicethereto being limited to that value which will effectively produce justsufllcient desensitization of the latter device to prevent -its falseoperation by applied signal echoes.

ANDREW C. NORWDIE.

